Ceramic circuit boards or substrates are often used in the manufacture of electronic equipment due to their ruggedness and resistance to environmental extremes. For example, ceramic circuit boards are commonly used in the manufacture of hybrid circuits. Hybrid circuits are employed in a wide range of devices, such as electronic test equipment, computers, and electronic equipment for aircraft, just to name a few. Several other uses for ceramic hybrid circuits are shown and described in the October 1993 issue of Semiconductor International at page 56 in an article entitled "Will Hybrid Circuits Survive?", which is hereby specifically incorporated by reference for all that it discloses.
Hybrid circuits of the kind described above typically comprise a ceramic substrate to which are mounted a variety of integrated circuit chips. The various integrated circuit chips are electrically connected to one another by a plurality of conductive metallic traces (printed circuits) deposited directly on the substrate. In most cases, the substrate also includes passive electronic components, such as resistors and capacitors, required to make the circuit functional. The passive circuit elements are usually manufactured directly on the ceramic substrate by any number of well-known processes. Thick film screen printing is a common technique for forming circuit traces as well as passive electronic components of hybrid circuits.
It is also common to deposit a thin layer of metal on the back side of the substrate to act as a ground plane. The appropriate ground points on the conductive traces on the front side of the substrate are connected to this ground plane on the back side of the substrate by electrically conductive holes, also commonly referred to as vias.
Once manufactured, a ceramic circuit board is usually attached to a larger printed circuit board which may host a number of ceramic circuit boards having various functions. The printed circuit board electrically connects the ceramic circuit boards with each other and with other electronic components and circuits as necessary depending upon the requirements of the particular electronic device employing the circuit.
Printed circuit boards are usually constructed of an insulating epoxy-glass material in a well-known manner. Electrically conductive paths are provided on and within the printed circuit board to accomplish the necessary connections. Electrically conductive receptor sites are also provided for connecting with corresponding interconnect sites on the ceramic circuit boards.
This connection between the printed circuit board receptor sites and the ceramic circuit board interconnect sites represents a potential source of failures in electronic devices. Because the epoxy-glass printed circuit board material has a different coefficient of thermal expansion than the ceramic material of the ceramic circuit board, temperature cycling results in relative movement between the printed circuit board and the ceramic circuit board. Such relative movement produces stress in the connection, which usually consists of a soldered joint. Eventually, this stress will cause failure of the soldered connection, resulting in electrical discontinuity and thus failure of the electronic device.
One way of addressing this problem has been to provide compliant leads on the ceramic circuit board. These leads are soldered to the printed circuit board connection sites. Because the leads are flexible, they serve to absorb the relative movement between the printed circuit board and the ceramic circuit board. This type of ceramic circuit board is sometimes referred to as a "leaded chip carrier".
Although leaded chip carriers absorb relative movement induced by temperature cycling, they are relatively expensive to manufacture. They are also relatively bulky due to the length of the leads employed.
A more compact and economical carrier is known as a "leadless chip carrier". Leadless chip carriers are generally mounted directly to a printed circuit board. Instead of leads, these carriers have interconnect pads around the periphery of the carrier, see FIG. 1. These pads are then soldered directly to the printed circuit board receptor sites.
The soldering process generally entails the application of a metalized solder paste to the surface of the printed circuit board in locations where a solder connection is desired. Accordingly, the receptor sites on the printed circuit board would be coated with the solder paste. The ceramic circuit board, along with other desired components, are then placed on the printed circuit board at the appropriate locations. The entire package is then heated to the melting point of the solder material in order to accomplish a soldered bond in the desired locations.
Because leadless chip carriers are connected directly to printed circuit boards, they are particularly susceptible to thermal cycling induced failures as described above. This is particularly true for large area leadless chip carriers which are often required due to the complexity of the circuit being supported by the carrier.
Because of this thermal cycling problem, leadless chip carriers having areas of about 0.650 square inches and larger tend to be unreliable. For larger sizes, the more expensive and bulky leaded chip carriers are generally employed to ensure reliability of the circuit. This size related problem of leadless chip carriers is well recognized in the industry. See, for example, ELECTRONIC MATERIALS HANDBOOK--Volume 1 "Packaging" by ASM International, Materials Park, OH., 1989 ISBN: 0-89170-285-1 at page 205, which is hereby specifically incorporated by reference for all that it discloses.
Consequently, there is a need to address this thermal cycling problem to enable the reliable usage of larger area leadless chip carriers and to improve the reliability of smaller area leadless chip carriers.